feat(coding-agent): add experimental first-time setup flow (#5587)

Behind PI_EXPERIMENTAL=1, show a first-time setup dialog on interactive startup when the default agent directory is used and settings.json does not exist. The dialog preselects the detected terminal appearance with an explicit dark/light choice (live preview) and asks for opt-in analytics data sharing. Submitting writes settings.json, which serves as the completion marker; opting in stores a generated trackingId. Escape at any point skips out of setup.

packages/coding-agent/docs/settings.md

@@ -55,6 +55,8 @@ Use `/trust` in interactive mode to save a project trust decision for future ses
 | `defaultProjectTrust` | string | `"ask"` | Fallback project trust behavior: `"ask"`, `"always"`, or `"never"`. Global setting only |
 | `collapseChangelog` | boolean | `false` | Show condensed changelog after updates |
 | `enableInstallTelemetry` | boolean | `true` | Send an anonymous install/update version ping after first install or changelog-detected updates. This does not control update checks |
+| `enableAnalytics` | boolean | `false` | Opt-in analytics data sharing. Currently only asked for during the experimental first-time setup (`PI_EXPERIMENTAL=1`) |
+| `trackingId` | string | - | Analytics tracking identifier, generated when `enableAnalytics` is turned on |
 | `doubleEscapeAction` | string | `"tree"` | Action for double-escape: `"tree"`, `"fork"`, or `"none"` |
 | `treeFilterMode` | string | `"default"` | Default filter for `/tree`: `"default"`, `"no-tools"`, `"user-only"`, `"labeled-only"`, `"all"` |
 | `editorPaddingX` | number | `0` | Horizontal padding for input editor (0-3) |